1. Field of the Invention
The invention relates to a precision machining apparatus and a precision machining method used when grinding a body to be ground (hereinafter also referred to as “grinding target body”), such as a silicon wafer or a magnetic disc substrate in which precise geometric accuracy and evenness are required. More particularly, the invention relates to a precision machining apparatus and precision machining method that is able to precisely control both the thickness and evenness of a body to be ground by performing feedback control when controlling the posture of a posture control device and controlling the amount that a moving portion which forms the precision machining apparatus is moved, throughout all of the stages of grinding from rough grinding to precision grinding.
2. Description of the Related Art
Recently, there is an increasing demand for next-generation power devices to be smaller and have less energy loss. High precision and multiple layering of semiconductors used in electronics are examples of this. In order to meet these needs, it is possible to develop various machining methods such as a machining method for making semiconductor wafers, which are represented by the Si wafer, extremely thin, in which there is no dislocation and lattice distortion on or in the machining surface, and a machining method in which the surface roughness (Ra) is on the sub-nanometer to nanometer level and the evenness of the machining surface is on the sub-micrometer to micrometer level or less.
With an eye toward the automotive industry, the IGBT (Insulated Gate Bipolar Transistor) which is a power device for automobiles is a main part of inverter systems. In the future, the productivity of hybrid vehicles is expected to rise increasingly as the performance of these inverters increases and their size decreases. Therefore, Si wafers that form these IGBTs will need to be extremely thin, with a thickness of approximately 50 to 150 μm and preferably 90 to 120 μm, and the switching loss, constant loss, and heat loss will all need to be reduced. Moreover, the yield ratio in the electrode formation process of the semiconductors and the multiple layering of the semiconductors will improve by having the perfect surface that has no dislocations, lattice distortion, or other such defects on the machining surface, or the inside near the machining surface, of a circular Si wafer that has a diameter of approximately 200 to 400 mm, and by having the surface roughness (Ra) on the sub-nanometer to nanometer level and having the evenness on the sub-micrometer to micrometer level.
Typically, machining of the foregoing semiconductor currently requires multiple processes such as rough grinding using a diamond wheel, rubbing, etching, and Wet-CMP (Wet-Chemo Mechanical Polishing) using loose grain. In this conventional machining method, oxidized layers, dislocation, and lattice distortion can occur in the machining surface, making it extremely difficult to obtain a perfect surface. Also, the evenness of the wafer is also poor which results in a lower yield ratio due to damage of the wafers that occurs during machining or after electrode formation. Furthermore, with the conventional machining method, as the diameter of the wafer increases from 200 mm, to 300 mm, to 400 mm, it becomes difficult to make the wafer extremely thin. As a result, research is currently underway to make the thickness of wafers having a diameter of 200 mm on the 100 μm level.
In view of the foregoing problems with the conventional technology, the inventors thus disclose in Japanese Patent Application Publication No. 2000-141207 an invention relating to a precision flat surface machining machine which is able to efficiently and consistently perform, using only a precision diamond wheel, all of the processes from rough-machining to ultra-precision surface machining including a final ductility mode process.
There are three main movements that are important in the grinding process to which the diamond wheel is applied: rotation of the wheel, advancing or feeding the main spindle that supports the wheel, and positioning the body to be machined. Precisely controlling these movements makes precision machining possible. However, in order to consistently perform all of the processes from rough-machining to ultra-precision machining with a single apparatus, of the main movements described above, the advancing or feeding of the main spindle in particular must be controlled with extreme precision over a wide range. To control the main spindle in the conventional grinding process, a method which applied a servo motor, for example, is widely used. This method, however, is unable to provide control from the low pressure range to the high pressure range with sufficient precision. In particular, this method is unable to yield sufficient machining in the low pressure region in which ultra-precision machining is performed. The inventors therefore disclose in Japanese Patent Application Publication No. 2000-141207 a precision machining machine which performs pressure control by combining a servo motor with a super magnetostrictive actuator. The servo motor and a piezoelectric actuator are used in a pressure range of 10 gf/cm2 or greater and the super magnetostrictive actuator is used in a pressure range of 10 gf/cm2 to 0.01 gf/cm2. As a result, all of the processes from the rough machining to the ultra-precision machining can be performed consistently with a single apparatus. Also, the grinding wheel is a diamond cup wheel with a grit number finer than 3000.
It is important to check during the grinding process of the wafer surface of the semiconductor to make sure that the wafer has the desired thickness and that the wafer surface has the desired evenness not only when the product is finished but also in the grinding stage. Even though the grinding apparatus is controlled by a high performance actuator, as described above, checking the actual evenness and the like of the ground surface and feeding back the results to the next grinding process enables better machining accuracy to be obtained. Japanese Patent Application Publication No. JP-A-8-174417 discloses an invention relating to a grinding method which calculates the grinding amount from the difference in a disc-shaped sample thickness before and after grinding and keeps the grinding amount within an allowable range while changing the grinding conditions or replacing the grinding pad or the like when the calculated grinding amount deviates from an acceptable value.
In polishing or grinding control of a typical wafer, as described in Japanese Patent Application Publication No. JP-A-8-174417 above, the method typically employed calculates the grinding amount from the difference in wafer thickness before and after grinding and performs machining while determining whether the grinding amount is within the allowable range. However, as described in Japanese Patent Application Publication No. 2000-141207, with a grinding apparatus that can perform grinding from rough machining to ultra-precision machining consistently by controlling the pressure while grinding using a combination of a servo motor with a super magnetostrictive actuator, calculating the difference and the like in the wafer thickness before and after machining interferes with the speed that can be expected from grinding consistently from rough machining to ultra-precision machining. Also, when grinding from rough machining to ultra-precision machining, it is extremely difficult to precisely control the surface machining precision, thickness, and evenness and the like of the wafer from the amount of grinding. That is, when the grinding precision at the end of the rough machining is to be determined by the amount of grinding, it is not possible to determine whether the evenness (i.e., the degree of evenness of the overall wafer; evenness decreases if the overall wafer brakes or the wafer warps toward the center portion or edge portion) of the wafer is the desired evenness. As a result, in the ultra-precision machining stage which focuses on making minute adjustments to the thickness and evenness, minute adjustments may occasionally not be sufficient.